1. Field of the Invention
The invention relates generally to LED lamps and LED lighting, and more particularly to LED lamps suitable to replace a fluorescent lamp in a luminaire having a ballast for use with fluorescent lamps.
2. Description of the Related Art
Fluorescent lighting has been around for many years now. This form of lighting started out as a highly efficient alternative for incandescent light bulbs, but has recently been surpassed by LED lighting to some extent in terms of efficiency and power consumption, and also in other aspects as set out below.
Fluorescent lamps generally comprise a tube filled with an inert gas and a small amount of mercury, capped at both ends with double pinned end caps. The end caps contain a glow wire to preheat the gasses inside the tube and to vaporize the mercury in order to assist with ignition of the fluorescent lamp. Once the fluorescent lamp is ignited, heat generated by the conducted current keeps the fluorescent lamp in operational condition. To facilitate these starting conditions and to limit current through the fluorescent lamp during operation, and thus limit the power consumed, an electrical ballast is connected between the mains power supply and the fluorescent lamp and power is supplied to the lamp via the ballast.
When first introduced, the only available ballasts were simple inductive elements placed in series with the power supply to the fluorescent lamp, which limit consumed power by limiting the AC current as a result of the frequency dependent impedance of the inductor. An undesirable result is a relatively low power factor and relatively high reactive power. These types of ballasts are usually referred to as magnetic ballasts
More recently electronic ballasts have been introduced. Such electronic ballasts usually first convert AC mains power into DC power, and subsequently convert the DC power into high frequency AC power to drive the fluorescent lamp. The more recent electronic ballasts actively control current through the fluorescent lamp and actively control AC power absorbed by the ballast itself. This allows the system to have a power factor close to a value of one. Even though power absorbed by the electronic ballast and fluorescent lamp combined is only slightly lower than a system with a magnetic ballast, reactive power is greatly reduced. The efficiency of the ballast itself is also improved.
Although LED lighting itself is only slightly more efficient than fluorescent lighting, it has many other advantages. For example, no mercury is required for LED lighting, LED lighting is more directional, LEDs require less effort to control or regulate power consumed, and the lifetime is greatly increased over fluorescent lighting.
Thus, replacing existing fluorescent lighting systems with LED lighting systems is often desirable. However, costs for such replacement are relatively high. Replacement LED lamps which lack appropriate circuits to deal with the ballast cannot be inserted in luminaires designed for fluorescent lamps due to the ballast, so the existing luminaire for fluorescent lamps needs to be replaced. As a consequence, many users simply replace failed fluorescent lamps with another fluorescent lamp, even in view of the evident advantages of LED lamps. The incentive to replace fluorescent lamps with LED lamps is further diminished when only a single fluorescent tube in a multi-tube luminaire has failed. Replacing the luminaire would result in discarding fluorescent tubes still in functioning order.
Consequently, there is a need for an LED lamp that can be put into operation when mounted in an existing luminaire designed for a fluorescent lamp.
Currently, there are LED lamps on the market shaped like fluorescent tubes that can be placed in an existing luminaire. However, these LED lamps require the luminaire to be stripped of the ballast and re-wired to directly connect the LED lamp to a mains power supply without intervention of a ballast. The labour required for the stripping and re-wiring the luminaire negates much, if not all, of the savings involved in switching to LED lighting, or even presents higher costs.
Consequently, a replacement lamp that does not require modification of the luminaire is preferred. Previously the design of LED lamps would have to be modified in terms of electronics to allow the new LED lamps to be inserted in the lamp holders of an existing fluorescent luminaire and subjected to the influence of a magnetic or electronics ballast, even when it is often unknown in advance whether a luminaire comprises an older magnetic inductor based ballast or a more modern electronics based ballast.
Examples of possible configurations are shown in FIG. 1 for a magnetic ballast 5 and in FIG. 2 for an electronic ballast 6, connected to mains power supply 7. The LED lamp 1 comprises LEDs 2 and an LED driver circuit 3, as well as safety devices 4 to secure proper functioning of the LED driver. Such safety devices 4 ensure that prior fluorescent lamps are connected on both sides to a combination of the ballast 5, 6 and mains power 7, before circuits are actually established for lighting the fluorescent lamp.
This approach entails a two-stage conversion of the power to (at least approximately) again obtain mains power supply for the LEDs 2. The first conversion is performed by the ballast 5 or 6 and the second conversion is performed by the internal LED driver 3 in the LED lamp 1. With respect to regulating power to the LEDs, a conversion step or stage by the LED driver 3 in the LED lamp 1 should be at least approximately inverse to the transfer characteristics of the ballast 5 or 6, requiring two operating modes of the LED driver 3 for an electronic ballast as the precise type of ballast (a magnetic inductor based ballast or electronic ballast) is normally not known when inserting a LED lamp 1 into an existing luminaire.
It is highly desired to be able to manufacture a single type of LED lamp, and not several types to comply with the type of ballast, which would also avoid the problem of having to determine the type of ballast before purchasing a replacement LED lamp of the required type. The preferably uniform LED driver 3 of FIGS. 1 and 2 would be required to detect the type of ballast actually arranged in the luminaire and operate differently for different types of ballast, adding to the complexity, cost and inefficiency of the resulting configuration, at least in terms of manufacture, in order to provide this selection in dependence of the type of ballast.
The present invention addresses the above problems.